A biosensor is a sensor that utilizes the molecule-identifying function of a biological material, e.g. a microorganism, enzyme, antibody, DNA, and RNA, and applies such a biological material as a molecule-identifying element. In other words, the biosensor utilizes the reaction occurring when an immobilized biological material identifies a target substrate, oxygen consumed by breathing of living organisms, enzyme reaction, luminescence, and the like. Among biosensors, practical use of enzyme sensors is developing. For example, enzyme sensors for glucose, lactic acid, uric acid, and amino acid find applications in medical instrumentation and food processing industry.
In an enzyme sensor, for example, electrons generated by the reaction of a substrate contained in a sample liquid, i.e. an analyte, with an enzyme or the like reduce an electron acceptor and a measuring device electrochemically measures the amount of the reduced electron acceptor. Thus, quantitative analysis of the analyte is performed. An example of such a biosensor is a sensor proposed in Patent Application No. PCT/JP00/08012.
In this biosensor, as shown in FIG. 4, electrically insulated board 1 made of polyethylene terephthalate or other materials has measuring electrode 2 (also referred to as a “working electrode”), counter electrode 3, and detecting electrode 4 that are made of electrically conductive materials and formed in proximity to one another on the electrically insulated board. Formed on these electrodes is regent layer 5 that contains an enzyme specifically reacting with a particular component in the sample liquid, an electron carrier, a water-soluble polymer, and the like.
Laminated thereon and bonded thereto are spacer 6 having a notch for forming analyte feed passage 7, and cover 8 (second electrically insulated board) having air vent 9. One end of the notch in spacer 6 is in communication with air vent 9 provided through cover 8.
Described hereinafter is a system of checking for suction of an analyte when the content of a substrate in a sample liquid, i.e. the analyte, is determined using a conventional biosensor of such a structure.
First, a sample liquid is supplied to the inlet of analyte feed passage 7 while a constant voltage is applied between counter electrode 3 or measuring electrode 2 and detecting electrode 4 by a measuring device (not shown) coupled to the biosensor. The sample liquid is sucked into analyte feed passage 7 by capillarity, passes over counter electrode 3 and measuring electrode 2, and reaches detecting electrode 4. Then, dissolution of reagent layer 5 starts. At this time, the measuring device detects electrical changes occurring between counter electrode 3 or measuring electrode 2 and detecting electrode 4 and starts measuring operation.
However, such a biosensor has a problem. Counter electrode 3, measuring electrode 2, and detecting electrode 4 are disposed in proximity to one another. Thus, when an amount of sample liquid insufficient to fill analyte feed passage 7 is supplied as shown in FIGS. 5 and 6, for example, the sample liquid reaches detecting electrode 4 without completely covering measuring electrode 2 and then the measuring operation starts. This makes the response value lower than that given when the analyte feed passage is sufficiently filled with the sample liquid as shown in FIG. 7, thus deteriorating the performance of the biosensor. In the top views of FIGS. 5 through 7, reagent layer 5 is not shown for simplicity.
The present invention aims to address the above-mentioned problem. Therefore, it is an object of the present invention to improve accuracy of detecting the analyte by adding new ideas on the position and shape of the detecting electrode and to provide a high-performance biosensor having excellent accuracy of measurement.